FORMALDEHYDE 67 sulphate. As impurities it may contain especially copper, zinc, man- ganese, magnesium, arsenic, ferric salts and free sulphuric acid. 1. Copper, Zinc.—3 grams are dissolved in water, boiled with nitric acid, and precipitated with excess of ammonia ; after filtration, the nitrate is tested for copper and zinc by the ordinary methods. 2. Ferric Salts.—The sample is dissolved in recently boiled water, acidified with a little hydrochloric acid and tested with ammonium thio- cyanate. 3. Free Sulphuric Acid.—The aqueous solution, prepared with boiled water, should not redden blue litmus paper. 4. Other Impurities.—3 grams are dissolved in water and oxidised by boiling with nitric acid; after precipitation with ammonia and filtra- tion, the lime, magnesia, arsenic and any other impurities are sought in the filtrate. FORMALDEHYDE CH20 = 30 This is sold in aqueous solution under the name Formal or Formalin and is a colourless liquid with a peculiar, irritating odour, D = about 1-08 ; it contains 35-40 grams of formaldehyde per 100 c.c. The commercial product may be contaminated with formic and acetic acids, methyl alcohol, acetone, chlorides, sulphates, copper and calcium; its value depends essentially on the content of formaldehyde. 1. Acidity.—Tested with litmus paper ; an acidity corresponding with a drop of N-alkali per I c.c. of the sample is allowable. 2. Sulphuric Acid, Hydrochloric Acid, Metals.—These are detected in the I : 5 solution in the usual way. 3. Methyl Alcohol.—50 c.c. are treated with 100 c.c. of 10% ammonia, with which the formaldehyde combines, forming hexamethylenetetramine ; after a rest of 6 hours in a closed vessel, the greater part of the liquid is distilled off. The distillate is acidified with a slight excess of dilute sulphuric acid and redistilled, the temperatures at which the first fractions distil being observed: in presence of methyl alcohol, these will come over at about 66° and may be tested for methyJ alcohol (see Spirits : Detection of Denaturants). By collecting 50 c.c. of distillate (after acidification) and determining its specific gravity, the quantity of methyl alcohol present may be found approximately from the table on p. 40. 4. Acetone.—A few c.c. are treated with sodium hydroxide and iodine solution : in presence of acetone, iodoform is formed immediately in the cold. 5. Determination of the Formaldehyde (lodornetric method of Fresenins and Grunhut).—25 c.c. are diluted with water to 500 c.c. and 5 c.c. of the solution (= 0-25 c.c. of substance) introduced into a bottle of about 200 c.c. capacity fitted with a ground stopper ; 30 c.c. of N-sodium hydroxide are then added rapidly from a graduated cylinder and, from a burette and with shaking, 50 c.c. of N/5-iodine solution (the liquid shouldl. It often exhibits yellowish spots of ferric oxide orpanto with a little water and poured into 200 o.c.